1. Field of the Invention
The present invention relates to a digital copy control technique employed in a system in which digital apparatuses, such as DVD (digital video disc) recording/reproduction apparatuses, digital VCR, and digital TV, are connected to each other via interfaces having the function of mutual authentication. In particular, this invention is concerned with such a digital copy control technique that effectively prevents unauthorized copying that is not intended by a creator or producer of data, while limitedly permitting digital copying of the data.
2. Prior Art
In a conventional digital data transmission system, digital recording/reproduction apparatuses, such as optical disc (e.g., DVD) reproduction apparatus, digital TV, digital VCR, and others, are connected to each other via an IEEE-1394 bus that serves as an intelligent interface, so that data contents, such as pictures and music, are transmitted and received between these digital apparatuses. When digital data is transmitted and received between these apparatuses, the system checks if each of the apparatuses operates in a manner as intended by the creator or producer who created the contents of the data, and, if any apparatus does not operate as intended by the creator, data transfer is prohibited, to thereby prevent the user from illegally copying the picture and./or music contents against the intention of the creator.
Digital main data to be transmitted contains copy control information called "CCI". The CCI consists of two bits, namely, "00", "10" or "11", where "00" indicates permission of free copying, "10" indicates permission of one-time copying, and "11" indicates prohibition of copying.
Upon transmission of digital data, the transmitter-side apparatus initially checks the CCI to determine its copy restriction level for the contents of the digital data, and also checks, on the IEEE-1394 bus, whether the receiver-side apparatus operates in a manner as intended by the content creator. If complete authentication is established between the receiver-side apparatus and the transmitter-side apparatus, the contents is encrypted and transmitted from the transmitter-side apparatus. In this case, where the CCI information of the digital data transmitted from the transmitter-side apparatus is, for example, "10", and the receiver-side apparatus is a sound recording apparatus, the CCI is rewritten into "11" and thus recorded during copying of the digital data. As a result, second and subsequent copying actions are prohibited, and thus only one-generation copying is realized.
As another method for preventing unauthorized copying by digital picture apparatuses, it has been proposed to use electronic watermark information (generally called "watermark"). In this method, watermark information may be directly added to a non-obvious or low-intensity portion of the waveform of a picture signal, or may be embedded into a particular frequency component of frequency conversion information of the original signal. By providing the watermark with information on permission/prohibition of copying, the watermark is able to indicate whether free copying is permitted, or only reproduction of the data is permitted, for example.
In the conventional copy control method using CCI as described above, however, when the receiver-side apparatus rewrites the CCI from "10" (indicating permission of one-time copying) into "11" (indicating prohibition of copying), for example, it is relatively easy to rewrite "10" into "00" (indicating permission of free copying) through two-bit manipulation, thus causing a problem that unauthorized copying may be done without difficulty.
In the method using watermarks, on the other hand, watermark information is distributed over a relatively wide range of main data containing picture and/or voice information, and it is therefore difficult for the receiver-side apparatus to rewrite this information. If the watermark information is to be rewritten on the user level, the receiver-side apparatus must be equipped with a considerably large circuit. This is advantageous over the use of CCI in terms of prevention of unauthorized or illegal copying. However, since the watermark cannot be easily rewritten as described above, it is rather difficult to permit only one-time copying by rewriting or changing the flag (i.e., watermark) as in the case where CCI is used.